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Date: Wed, 06 Jun 2001 12:16:30 +1000
From: Andrew Morton <andrewm@uow.edu.au>
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To: "Jeffrey W. Baker" <jwbaker@acm.org>
CC: Derek Glidden <dglidden@illusionary.com>, linux-kernel@vger.kernel.org
Subject: Re: Break 2.4 VM in five easy steps
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"Jeffrey W. Baker" wrote:
> 
> Because the 2.4 VM is so broken, and
> because my machines are frequently deeply swapped,

The swapoff algorithms in 2.2 and 2.4 are basically identical.
The problem *appears* worse in 2.4 because it uses lots
more swap.

> they can sometimes take over 30 minutes to shutdown.

Yes. The sys_swapoff() system call can take many minutes
of CPU time.  It basically does:

	for (each page in swap device) {
		for (each process) {
			for (each page used by this process)
				stuff

It's interesting that you've found a case where this
actually has an operational impact.

Haven't looked at it closely, but I think the algorithm
could become something like:

	for (each process) {
		for (each page in this process) {
			if (page is on target swap device)
				get_it_off()
		}
	}

	for (each page in swap device) {
		if (it is busy)
			complain()
	}

That's 10^4 to 10^6 times faster.

-